Laser printing of label materials is a relatively new development and employs apparatus such as shown and described in Gretchev, U.S. Pat. No. 4,494,129. The high pressures and temperatures employed in laser printing can be accommodated by simple, non-composite paper and synthetic sheets. They will not curl and become difficult to feed and stack. If, however, dual-layer and multi-layer composite sheets, which are necessary components of label or tag sheets, are fed to laser printers, special methods are needed to prevent the composite sheets from curling during and after printing. In one such method, Rutkowski, U.S. Pat. No. 4,913,926, discloses laser printing a dual-layer label sheet having a continuous film of pressure sensitive adhesive between the layers. The resultant sheet is shown to be curled up after passing through the printer. If, however, the adhesive is patterned into geometric figures, such as hexagons and diamonds, during deposition to leave a number of gaps between the patterns, and then the sheets are laser printed, no curling is observed because the melted and squeezed adhesive layer flows into and fills the gaps relieving any stresses developed by heat and pressure. In the method of Rutkowski, special equipment will be necessary for patterned printing and the high degree quality control necessary should be labor intensive. Another approach to providing adhesive paper labels having curling resistance and applicable to electrostatic and magnetic copying in disclosed by Fuji Xerox Corp in Japanese Patent Publication No. J59149970-A, Aug. 28, 1984. Such a composite comprises two sheets, a surface paper to receive the printing and a release paper having a ratio of surface paper to release paper elongation and/or contraction of 0.7-1.0:1.0. Furthermore, the release paper has an elongation and/or contraction of below 0.90%. If such a composite is used to make laser printed labels comprising wood pulp paper, curling is distinctly not a problem, but the labels do not weather well and cannot be used outdoors, such as to mark lumber, because rain, wind and snow will make them difficult to read and maintain. If, on the other hand, such labels are made of synthetic paper, such as polyvinyl chloride paper, curling is not a problem because the elongation/contraction requirements are met, but the vinyl paper, like wood pulp paper, doesn't weather well and tears readily during application, and has a tendency to give off dangerous gases if burned. Improved weatherability, equivalent printability, high tear resistance and no tendency to elaborate noxious gases during heating can be achieved, if instead of vinyl, tear-resistant synthetic papers are used, such as those based on thermoplastic polyesters and polyolefins, e.g., polypropylene, and the like. However, the substitution of these for wood pulp paper and vinyl paper in the composite dual-layer label sheets of the Rutkowski and the Japanese Publication, above-mentioned, lead to serious and substantial curling problems during and after laser printing.
It has now been discovered that if a composite label sheet is provided having at least three layers and if, further, the top and bottom layers are selected to have the same or substantially the same thermal expansion and contraction characteristics, then non-vinyl, tear-resistant plastic papers such as polyester and polyolefins and the like can be used to provide non-curling label sheets, with none of the above-mentioned disadvantages of wood pulp paper and vinyl composite sheets and labels. This result is unexpected in view of the art because the base layer of this invention can have an expansion/contraction ratio substantially different than either the printable top layer and the protective backing layer or coating, whereas the Japanese Patent Publication would teach otherwise.
Accordingly, a principal object of the present invention is to provide laser printable sheets of labels or tags mounted on backing sheets without the curling problem discussed above. It is a further object of the invention to provide a method for laser printing sheets of labels mounted on protective backing without curling. It is still another object of the invention to provide articles labeled with curl-free laser printed labels or tagged with curl free laser printed tags.
These and other objects of the invention will become apparent from the present specification and drawing.